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Absorption – Definition, Mechanisms and Disorders

Absorption is the physiological process by which nutrients, fluids, or active substances are taken up through biological membranes and transported into the bloodstream or lymphatic system.

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Things worth knowing about "Absorption"

Absorption is the physiological process by which nutrients, fluids, or active substances are taken up through biological membranes and transported into the bloodstream or lymphatic system.

What is Absorption?

Absorption refers to the physiological process by which substances such as nutrients, fluids, electrolytes, and drug compounds pass through biological membranes and enter the bloodstream or lymphatic circulation. The term is used across multiple medical disciplines, including gastroenterology, nutritional medicine, and pharmacology.

Sites of Absorption in the Body

Absorption occurs at several locations throughout the body. The most important site for nutrient absorption is the small intestine, particularly the jejunum and ileum. Other key sites include:

Stomach: limited absorption of water, alcohol, and certain medications
Large intestine: absorption of water and electrolytes
Oral mucosa: sublingual absorption of specific drugs (e.g., nitroglycerin)
Skin: transdermal absorption of active substances (e.g., patches)
Lungs: absorption of inhaled substances

Mechanisms of Absorption

Depending on the substance and location, absorption occurs via several transport mechanisms:

Passive diffusion: substances move along a concentration gradient through the cell membrane without energy expenditure
Active transport: energy-dependent uptake of substances against a concentration gradient, e.g., for glucose and amino acids
Facilitated diffusion: transport through specific carrier proteins without energy use
Endocytosis (pinocytosis): uptake of macromolecules through membrane invagination

Absorption of Nutrients

Carbohydrates

Carbohydrates are enzymatically broken down into simple sugars (monosaccharides such as glucose and fructose) before being absorbed into intestinal cells via active or facilitated transport. They are then transported via portal blood to the liver.

Proteins

Proteins are cleaved by enzymes called proteases into amino acids and small peptides, which are actively transported into enterocytes (intestinal cells) and carried into the bloodstream.

Fats

Fats are emulsified by bile acids and broken down by lipases. The resulting fatty acids and monoglycerides are taken up into intestinal cells, reassembled into triglycerides, and transported as chylomicrons via the lymphatic system into the bloodstream.

Vitamins and Minerals

Fat-soluble vitamins (A, D, E, K) are absorbed together with dietary fats. Water-soluble vitamins (e.g., vitamin C and B vitamins) are absorbed via specific transporters. Minerals such as calcium, iron, and zinc are subject to regulated absorption that depends on the body's current status and requirements.

Absorption Disorders

When absorption is impaired or insufficient, the condition is referred to as malabsorption. Common causes include:

Small intestinal diseases (e.g., celiac disease, Crohn's disease)
Pancreatic insufficiency with lack of digestive enzymes
Bile acid deficiency or biliary tract disease
Post-surgical conditions (e.g., short bowel syndrome)
Intestinal infections or parasitic conditions

Consequences may include malnutrition, weight loss, and deficiencies in essential vitamins and minerals.

Absorption in Pharmacology

In pharmacology, absorption describes how and how quickly a drug moves from its site of administration into the bloodstream. Bioavailability refers to the fraction of an administered dose that reaches systemic circulation in an active form. It is influenced by the route of administration (oral, intravenous, inhalation, etc.), the pharmaceutical formulation, the physicochemical properties of the drug, and the first-pass effect in the liver following oral administration.

References

Schmidt, R. F., Lang, F., Heckmann, M. (eds.): Physiologie des Menschen. 31st edition. Springer Medizin Verlag, Heidelberg 2010.
World Health Organization (WHO): Nutrient absorption and bioavailability. WHO Technical Report Series. Geneva 2004.
Rang, H. P., Dale, M. M., Ritter, J. M., Flower, R. J.: Rang and Dale's Pharmacology. 7th edition. Churchill Livingstone, Edinburgh 2011.

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